We recently had a situation where a water line burst and some of the falling water made it's way to above one of our MCCs. I shut off the 600 amp main breaker, in order to de-energize the MCC. Typically our load is 200-300 amps (@ 480Y/277). Someone suggested that I should have shut off the individual breakers prior to opening the main. I see no reason do this, particularly in this situation, however I would like opinions on opening the main (not necessarily regularly) under it's rated load.
I agree. If I felt there was time, I would probably dump some branch loads first, not for the sake of the main CB, but for the sake of sensitive equipment out there somewhere that might not like being hammered.
With larger loads, dumping some stuff makes it more likely that certain types of switches will open and reduces the arc intensity when they are opened.
Re: Opening breaker under rated load.#2771 07/24/0103:37 PM07/24/0103:37 PM
Wow, where i'm from I'd never even think about turning on a main without shedding the load first. You can break down a large inrush current by bringing on the loads a little bit at a time, especially with motor loads.
Re: Opening breaker under rated load.#2773 07/24/0107:14 PM07/24/0107:14 PM
Originally posted by CanadianSparky: Wow, where i'm from I'd never even think about turning on a main without shedding the load first. You can break down a large inrush current by bringing on the loads a little bit at a time, especially with motor loads.
I understand your point and agree fully. Inrush can be minimized by bringing the load on gradually. I was discussing turning the power off, though.
Re: Opening breaker under rated load.#2775 07/24/0110:39 PM07/24/0110:39 PM
Originally posted by sam: I believe you would have to make a really Quick judgement call on that situation. Hope no one was injured in this incident!
Luckily, nobody was hurt and equipment damage was minimal. I would have preferred to shut the feeder off at the substation supplying our building. However, up until this incident, we did not have a key. I had to decide quickly what to do.
Re: Opening breaker under rated load.#2777 07/25/0101:16 PM07/25/0101:16 PM
gelnn35 is right and i belive pissing around de-energizing the load side of the MCC while water is leaking into the enclosure would be the worst course of action.ABOVE ALL THE MAIN CIRCUIT BREAKER BY FAR IS THE SAFEST PLACE TO DE-ENERGIZE OR ENERGIZE ANY LOAD THAT COULD COME UNDER A FAULT CURRENT CONDITION.The load at risk included the line side of the MCC.
[This message has been edited by frank (edited 07-26-2001).]